1. Field of the Invention
This invention relates generally to suspension systems and methods for isolating and reducing the transmission of vibratory motion between an object and a base and, more particularly, to an omnidirectional vibration isolation or suspension system that exhibits low stiffness, high damping to limit resonant responses of the system, effective isolation at the higher frequencies, and can accommodate changing weight loads without significantly degrading isolation system performance. The present invention can also be retrofitted to existing vibration-isolating suspension systems to improve their performance.
2. Description of Related Art
The problems caused by unwanted vibration on motion-sensitive equipment and devices have been widely researched and numerous solutions to prevent or reduce the transmission of vibratory motion have been proposed and developed. Many of the devices designed to reduce the transmission of unwanted vibration between an object and its surroundings, commonly called vibration isolators or suspension devices, have utilized various combinations of elements such as resilient pads made from a variety of materials, various types of mechanical springs, and pneumatic devices. There are, however, serious shortcomings and disadvantages associated with these particular prior art isolation systems which prevent them from obtaining low system natural frequencies and from limiting resonant responses to low values while providing high isolation performance at the higher frequencies.
These shortcomings and disadvantages of prior art systems were addressed through my development of novel vibration isolation system described in my co-pending application Ser. No. 395,093, filed Aug. 16, 1989, entitled "VIBRATION ISOLATION SYSTEM", and my co-pending application, Ser. No. 681,808, filed Apr. 8, 1991, entitled "DAMPED VIBRATION ISOLATION SYSTEM", which are both hereby incorporated by reference in this present application. The particular vibration isolation system described in my co-pending applications and utilized in connection with the present invention provides versatile vibration isolation by exhibiting low stiffness in an axial direction (generally the direction of the payload weight) and any direction substantially transverse to the axial direction (generally a horizontal direction). The particular system utilizes a combination of isolators that can be connected together axially in series to provide omnidirectional isolation. Each isolator is designed to isolate either the axial or the transverse component of any vibratory motion to effectively isolate vibrations in all directions. In subsequent discussions, an axial-motion isolator will be referred to as a vertical-motion isolator, and the system of axial-motion isolators will be referred to as the vertical-motion isolation system. Similarly, a transverse-motion isolator will be referred to as a horizontal-motion isolator, and the system of transverse-motion isolators will be referred to as the horizontal-motion isolation system.
Each isolator relies on a principle of loading a particular elastic structure which forms the isolator or a portion of it (the loading being applied by either the supported weight or by an external loading mechanism) to approach the elastic structure's point of elastic instability. This loading to approach this point of elastic instability, also called the "critical buckling load" of the structure, causes a substantial reduction of either the vertical or the horizontal stiffness of the isolator to create an isolation system that has low stiffness (that can be made zero or near zero) in the vertical and in any horizontal direction, and increases the damping inherent in the structure. While stiffness is reduced, these isolators still retain the ability to support the payload weight.
If the load on an elastic structure with an instability is greater than the critical buckling load, the excessive load will tend to propel the structure into its buckled shape, creating a "negative-stiffness" or "negative-spring-rate" mechanism. By combining a negative-stiffness mechanism with a spring, adjusted so that the negative stiffness cancels or nearly cancels the positive stiffness of the spring, one obtains a the resulting device that can be placed at or near its point of elastic instability. The magnitude of the load causing the negative stiffness can be adjusted, creating an isolator that can be "fine tuned" to the particular stiffness desired.
Many applications of my vibration isolation system involve changing weight loads. Examples are optical tables that support optical instruments of various weights and various weight distributions on the tables; photolithography machines with stages that translate and change the weight distributions of the machines; and isolated floors that support equipment of various weights and various weight distributions. It is thus desirable for the isolation system to accommodate changes in the weight distributions passively, with minimum added complexity and with minimum degradation in isolation system performance.
In order to maintain the performance of the isolation system under the changing weight loads, it is also desirable for the stiffness of the isolators to change in some prescribed manner with change in weight load. For example, if the isolation system stiffness changes in the same proportion as changes in the payload weight, the system resonant frequency remains unchanged, and the resulting transmissibility vs. frequency ratio curve remains unchanged. As another example, consider a "CG system", i.e., a vibration-isolating suspension system in which the resultant of the isolator forces passes through the center of gravity of the payload. For such a system, a purely translational excitation produces a purely translational response. This configuration is desirable because it minimizes the payload response for a given excitation. (A "non-CG system" produces translational-rotational coupling, i.e., a purely translational excitation produces a combined translational and rotational response of the payload, with a maximum response greater than that of a CG system). In a CG system, if the isolator stiffnesses change in the same proportion as the changes in weight supported by each isolator, the system tends to remain a CG system under a changing payload weight distribution. Accordingly, for many systems it is desirable that the stiffness of an isolator increase in the same proportion as changes in the payload weight supported by the isolator.
Many existing vibration-isolating suspension systems could benefit from reduced suspension system stiffness in order to reduce system natural frequencies and improve isolation system performance by isolating at lower frequencies and by isolating more effectively at higher frequencies, or that could benefit from increased damping in order to reduce isolation system resonant responses, or that could benefit from both reduced stiffness and increased damping. Accordingly, there is a need for an improved means for practically and effectively retrofitting existing vibration-isolating suspension systems in order to reduce the system stiffness or to increase the system damping, or to both reduce the system stiffness and increase the system damping. It is desirable that such retrofitting means would not disturb the existing system, or disturb it as little as possible by displacing it from its equilibrium position or by requiring changes in the existing system such as adding or removing weight.